Polyurethane foam sheets, and particularly polyurethane foam sheets produced using so-called flexible polyurethane as a raw material, exhibit excellent flexibility, elasticity, and cushioning characteristics, and are consequently used, in combination with various fabrics, in a variety of different applications such as clothing, boots, and supporters and the like. In conventional methods of producing these polyurethane foam sheets, a low boiling point organic solvent such as a chlorofluorocarbon (such as CFC-11 or CFC-113) or methylene chloride has typically been used as the foaming agent. However, in recent years, the toxicity of these organic solvents, together with environmental concerns, have resulted in ongoing restrictions or even prohibition of the production and usage of these solvents, and there is now a pressing need for a shift to a method of producing polyurethane foam sheets that does not require the use of such organic solvents.
Furthermore, in the present invention, the term laminated sheet refers to a sheet with a laminated structure including an aforementioned polyurethane foam sheet, as well as a nonwoven fabric, a woven fabric, or a knitted fabric or the like, and artificial leather and synthetic leather are typical examples. Polyurethane resins have conventionally been widely used within artificial leather and synthetic leather applications, and generally, these artificial or synthetic leathers employ a laminated structure that includes a foam layer (a porous layer) containing a polyurethane resin, which is used for imparting a leather-like texture.
“Artificial leather” is generally produced by a so-called wet process, wherein a dimethylformamide (hereafter also referred to as DMF) solution of a polyurethane resin composition is impregnated into, or coated onto, a nonwoven fabric, the polyurethane is then solidified within a water coagulation bath or a coagulation bath of a mixture of DMF and water, thereby forming a foam layer (a porous layer), and the fabric then passes through a washing step and a drying step.
Furthermore, “synthetic leather” is generally produced by the same wet process as that described above for “artificial leather”, with the exception of using a woven fabric or knitted fabric instead of the nonwoven fabric. Synthetic leather can also be produced by a so-called dry process, although this process also requires the use of a large quantity of organic solvent, and in this regard, differs little from the wet process.
In all of the above conventional methods of producing artificial leather and synthetic leather, an organic solvent-based polyurethane resin is used, and because the production process inevitably requires the drying or extraction of the organic solvent, a variety of problems arise, such as the deleterious impact on human health, environmental contamination, and the energy requirements associated with evaporating the organic solvent. Accordingly, the industry is now strongly demanding a shift from organic solvent-based resins to either water-based polyurethane resins or solvent-free polyurethane resins, and a shift to production methods that do not require the use of organic solvents.
For example, investigations into the use of water-based polyurethane resins instead of organic solvent-based polyurethane resins are already being conducted, but because the resulting polyurethane foam sheets, and laminated sheets of artificial leather or synthetic leather exhibit inferior water resistance and durability, their applicability is extremely limited.
Furthermore, a method of producing a foamed polyurethane elastomer sheet, wherein a prepolymer containing isocyanate groups at the terminals is mixed with 2-pyrrolidone, the resulting mixture is formed into a sheet, and the sheet-like mixture is then foamed and cured by contact treatment with steam has already been disclosed (see Japanese Unexamined Patent Application, First Publication No. Hei 11-246695 (page 3, left column, paragraph 0016 to page 4, right column, paragraph 0025)).
The prepolymer containing isocyanate groups at the terminals disclosed specifically within the above application is liquid at room temperature, and undergoes a cross-linking reaction with moisture (water) to form a solid, although if not mixed with 2-pyrrolidone, the prepolymer undergoes almost no foaming even in contact with steam, and it is only upon mixing with 2-pyrrolidone that a favorable level of foaming is achieved. Accordingly, the resulting foamed polyurethane elastomer contains residual organic solvent, namely the 2-pyrrolidone, within the elastomer, and can therefore not be considered solvent-free.
In addition, the above sheet-like mixture of the prepolymer and 2-pyrrolidone disclosed specifically within the above application is of low viscosity, and remains of low viscosity even after foaming by contact treatment with steam, meaning the foam shape is difficult to stabilize. Furthermore, because the progression of the subsequent cross-linking reaction caused by reaction with atmospheric moisture requires considerable time, and the viscosity increase of the above sheet-like mixture is inadequate, meaning the cohesive force is poor, when the foamed polyurethane elastomer sheet is wound into a roll or the like, the stress applied to the sheet can cause deformation or even collapse of the foam shape.
Moreover, when the foamed polyurethane elastomer sheet is laminated to another substrate to form a laminated sheet, components of the above mixture can penetrate into the other substrate, causing the texture of the resulting laminated sheet to become overly hard.
Techniques involving reactive hot melts are also being investigated as potential methods of producing polyurethane foam sheets and laminated sheets using solvent-free polyurethane resins. These reactive hot melts are solids at room temperature, but melt to form a liquid under heating, and combine a “hot melt property”, wherein the cohesive force is restored on cooling, with favorable “moisture curability”, wherein a cross-linking reaction caused by a reaction between isocyanate groups and moisture (water) ensures excellent levels of adhesion and durability (and particularly resistance to hydrolysis and heat). In recent years, reactive hot melts have been attracting considerable attention in a variety of fields as a potential method for removing the need for solvents.
For example, a method of producing a polyurethane porous sheet-like structure, wherein an isocyanate group-containing urethane prepolymer, which is either a semisolid or solid at room temperature but has been melted by heating, a compound capable of reaction with an isocyanate group, which is either heated or at room temperature, and/or a urethane curing catalyst are subjected to high-speed mixing while a gas is introduced to effect mechanical foaming, and the resulting foamed product is then spread out into a sheet-like structure has already been disclosed (see Japanese Unexamined Patent Application, First Publication No. 2002-249534 (page 3, left column, paragraph 0007 to page 5, right column, paragraph 0037)).
Using this method, polyurethane foam sheets and laminated sheets of a certain degree of thickness can be produced via a production process that requires no discharge of organic solvents into the environment, and also enables considerable energy conservation.
However, in those cases where the applied thickness of the sheet-like foam product is relatively thin, namely, from 50 to approximately 200 μm, the foam shape generated by mechanical foaming tends to be non-uniform, and the foaming degree of the foam is extremely small. In addition, because the progression of the cross-linking reaction of the isocyanate groups requires considerable time, when the polyurethane porous sheet-like structure is wound into a roll or the like, the stress applied to the sheet can cause deformation or even collapse of the foam shape.